Rubber compositions for fiber cord toppings are required to improve in the properties such as handling stability, fuel economy, elongation at break, adhesion to fiber cords, and processability in a balanced manner.
As a method for improving these properties, for example, Patent Literature 1 discloses the use of a crosslinkable resin such as a modified resorcin resin. However, there is an arising demand for the development of a technique that can reduce the use of crosslinkable resins in view of the cost and environmental concerns. Yet, a reduced use of crosslinkable resins unfortunately decreases adhesion to fiber cords, thus decreasing tire durability.
Meanwhile, typical rubber compositions for fiber cord toppings contain insoluble sulfur, and 80% by mass or more of the insoluble sulfur remains unconverted to soluble sulfur even after the rubber kneading process. Such insoluble sulfur is present in the form of particles suspended in the rubber composition, or in the form of being adsorbed on substances such as zinc oxide particles, carbon black, and silica. On the other hand, conversion of a large amount of insoluble sulfur to soluble sulfur before vulcanization would unfortunately cause blooming of sulfur to the surface of the fiber cord topping rubber, thus decreasing building tack to deteriorate processability (extrusion processability) and adhesion to fiber cords (in a brand new state and after wet heat degradation), and also causing bulging and separation in the tire to lower the tire durability. Thus, it is important to prevent blooming of sulfur to improve the above properties in a balanced manner. Blooming herein refers to a phenomenon where sulfur is migrated to and deposited on the surface of a rubber composition like blooming flowers.
A technique of using a large amount of zinc oxide has been generally employed to prevent such blooming of sulfur; however, the zinc oxide contained in rubber compositions for tires has been a recent concern in terms of environmental pollution (particularly, plant growth inhibition), and the use of a smaller amount of zinc oxide is thus desired.
As described above, there is a demand for a technique that can improve the handling stability, fuel economy, elongation at break, adhesion to fiber cords, processability, and tire durability in a balanced manner with smaller amounts of crosslinkable resins and zinc oxide.